The foamy viruses, or syncytium-forming viruses, are members of the spumaviriniae sub-family of retroviruses. Foamy viruses are found in many mammalian species and appear to be non-pathogenic in their natural hosts even though they have a wide tissue range and induce extensive cytopathology in cell cultures. The genomes of simian foamy virus type 1 (SFV-l) from rhesus macaque and human spumaretrovirus (HSRV) have been molecularly cloned and sequenced; pairwise comparisons of these two viruses show from 30% to 80% homology, depending on the region selected for analysis. Both viruses encode several open reading frames (ORFs) in addition to genes for virion structural proteins. Recent studies have revealed that SFV-1 and HSRV each encode a transactivator (taf) which functions to strongly augment transcription directed by the viral long terminal repeat (LTR). The main objective of this proposal is to elucidate molecular mechanisms regulating SFV-1 gene expression directed by the viral LTR. It remains to be determined whether taf acts directly on the LTR or whether transactivation is mediated by cellular factors. The target (TAR) for taf is distributed over two parts of the U3 domain of the viral LTR upstream from the TATA box in the viral promoter. Each these two TAR regions is about 300 base pairs (bp) long and no homologies (i.e., repeat elements) are detected between the two TARs. In addition, the promoter-distal TAR (TAR-d) functions in only in the sense orientation whereas the promoter-proximal TAR (TAR-p) functions in either orientation. These observations (on the complex nature of the targets) support the hypothesis that taf may function through more than one cellular factor which interacts with different sequence elements in the U3 region of the LTR. SPECIFIC AIM 1: Binding sites for cellular factors and the precise target sequences for taf in the SFV-1 LTR will be defined. SPECIFIC AIM 2: Monospecific antibodies will be made to detect the SFV-1 taf gene product; taf protein in infected cells will be extensively characterized. SPECIFIC AIM 3: Genetically engineered yeast and mammalian cells will be used to produce the SFV-1 taf gene product for structural and functional studies. SPECIFIC AIM 4: To elucidate the mechanism of SFV-1 transcriptional transactivation, functional domains of taf will be identified by mutagenesis and by evaluation of hybrid transactivators. These studies on SFV-1 regulation are relevant for elucidating mechanisms which control viral latency (or persistent infection) in the host animal. Observations made in the course of the proposed research will also enhance the understanding of mechanisms controlling eucaryotic transcription; the proposed studies on SFV-1 gene expression are significant since new (cellular) transcriptional factors may be identified.